usually they will only come on via a computer once they get toally discharged. mains chargers will not work as they sometimes need to detact a small voltage from the unit to start the charge. If its dead flat they won't charge it.

Update

I am talking from my own experience. I had a dead Nano and it would not charge from a PSU or dock, but did from a computer. Possibly it was because it was not an Apple psu. I would advise anyone with a dead Nano to try recharging from all sources.

sbdfhs, again your answer is incorrect. A computer USB output is no less than 4.75 V and no more than 5.25 V and 500mA. It will NOT charge a battery that is below 2.7V You WILL have to use at least an AC charger since the spces on those are Input 100-240V - 0.15A (0.15A) 50-60hz Output 5V --- 1A So your answer is very misleading. May be an edit would be nice:-)

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